Central venous access devices, which are essential in the care of many oncology patients, often become occluded with fibrin. While this obstruction can usually be removed with routine instillations of urokinase, this sometimes does not work satisfactorily. This project is studying the efficacy of continuous (1 to 12 hours) infusions of low-dose urokinase to restore function to access devices refractory to the routine urokinase instillations. Forty-one patients have been accrued with a goal of 100. The treatment has been successful in approximately 80% of cases. (Protocol #93-CC-0135)